1. Field of the Invention
This invention relates generally to three dimensional coordinate measuring machines (or CMM's). More particularly, this invention relates to a new and improved mounting device for a three dimensional CMM having a multijointed arm which allows for quick changes in the location of the arm with a high degree of dimensional repeatability.
2. Prior Art
It will be appreciated that everything in the physical world occupies volume or space. Position in a space may be defined by length, width and height which, in engineering terms, is often called an X, Y, Z coordinate. The X, Y, Z numbers represent the dimensions of length, width and height or three dimensions. Three-dimensional objects are described in terms of position and orientation; that is, not just where an object is but in what direction it points. The orientation of an object in space can be defined by the position of three points on the object. Orientation can also be described by the angles of alignment of the object in space. The X, Y, and Z coordinates can be most simply measured by three linear scales. In other words, if you lay a scale along the length, width and height of a space, you can measure the position of a point in the space.
Presently, coordinate measurement machines or CMM's measure objects in a space using three linear scales. FARO Technologies, Inc. of Lake Mary, Fla. (the assignee of the present invention) has successfully produced a series of electrogoniometer-type digitizing devices for the medical field. In particular, FARO Technologies, Inc. has produced systems for skeletal analysis known as METRECOM.RTM. and systems for use in surgical applications known as SURGICOM.TM.. Electrogoniometer-type devices of the type embodied in the METRECOM and SURGICOM systems are disclosed in U.S. Pat. Nos. 4,670,851, 5,251,127 and 5,305,203, all of which are assigned to the assignee hereof and incorporated herein by reference.
Portable CMM's are now used for three dimensional measurement of objects for reverse engineering, inspection, etc. An exemplary portable CMM system is disclosed in U.S. Pat. No. 5,402,582 assigned to the assignee of the present application, and which is incorporated herein by reference. As shown in prior art FIG. 1, the three dimensional measuring system of U.S. Pat. No. 5,402,582 generally comprises a coordinate measuring machine (CMM) 10 comprised of a manually operated multijointed arm 12 and a support base or post 14, a controller or serial box 16 and a host computer 18. It will be appreciated that CMM 10 electronically communicates with serial box 16 which, in turn, electronically communicates with host computer 18.
CMM 10 includes transducers (e.g., one transducer for each degree of freedom) which gather rotational positioning data and forward this basic data to serial box 16. Serial box 16 provides a reduction in the overall requirements of host computer 18 to handle certain complex calculations and provides certain preliminary data manipulations. The serial box 16 includes EEPROMS which store data control software, a microcomputer processor which manipulates data, a signal processing board for communicating data and a number of indicator lights 20. As mentioned, basic transducer data is sent from CMM 10 to serial box 16. Serial box 16 then processes the raw transducer data on an ongoing basis and responds to the queries of the host computer with the desired three-dimensional positional or orientational information.
As shown in prior art FIG. 2, the CMM 10 comprises a base 14 connected to a first set of two transfer housings including a first transfer housing 40 which, in turn, is connected to a second transfer housing 42 (positioned transverse to housing 40). A first extension member 44 is rigidly attached to a second set of two transfer housings including a third transfer housing 46 transversely attached to a fourth transfer housing 48. First extension member 44 is positioned perpendicularly between transfer housings 42 and 46. A second extension member 50 is aligned with and rigidly attached to transfer housing 48. Rigid extension member 50 is rigidly attached to a third set of two transfer housings including a fifth transfer housing 52 transversely attached to a sixth transfer housing 54. Fifth transfer housing 54 has attached thereto a handle/probe assembly 56 at the end of the multijointed arm. Additional details of the CMM 10 are disclosed in U.S. Pat. No. 5,402,582 which is incorporated herein by reference.
While the CMM systems of the prior art are well suited for their intended purposes, there are, however, drawbacks to the prior art with respect to the mounting system to affix arm 12 on its support, e.g., portable stand, table, wall, fixture, jig, ceiling, etc. There is often a need to change the location of a CMM system for a new process or use of the system. Changes in location may be include changes in physical location or orientation for a new use or process. The prior art mounting system, not shown in prior art FIG. 1 or 2, utilizes a six hole mounting plate attached to arm 12 and six bolts for securing the mounting plate to various adaptors. The deficiency of this design is that the six bolts are time consuming to install and in the event arm 12 is mounted overhead or on the side of a wall, significant effort is required to hold arm 12 in position while the bolts are being bolted into position. It is also advantageous that changes in location afford a high degree of dimensional repeatability so that the need to set up a new coordinate system after the moving the arm is substantially reduced or eliminated. Though the prior art provides a high degree of dimensional repeatability due to the number of bolts referencing the mounting plate, installation using the bolts is both cumbersome and time consuming. Thus there is a need in the industry for a mounting system which allows for quick changes in the location of the CMM with a high degree of dimensional repeatability.